The most commonly used form of printed circuit board is one having a tin/lead coating fused over copper. This type of board has proven adequate as long as the board is to be hand soldered, but has been undesirable when soldered in a wave soldering machine, since many conductors in the connection pattern on the board are thus inadvertently soldered together, forming improper circuit connections. To overcome this problem with wave soldering, subsequent prior art printed circuit boards have applied a solder resist mask over any area of the board, including the tin/lead coated copper circuitry, which was to be protected from solder in subsequent fabrication steps. This solder mask over tin/lead technique has proved satisfactory in certain applications in that it eliminated the majority of the shorting problems and unintentional connections. However, because the mask was applied over the tin/lead coating on the circuitry, when the board was later subjected to the wave of molten solder, the mask frequently broke off the circuit areas intended to be protected since the tin/lead supporting the mask could melt under the temperature of the solder and thus no longer provide mechanical support to the mask.
Due to the disadvantages of the mask-over-tin/lead technique, a subsequent technique of applying the solder mask directly over the bare copper was developed. In the fabrication of this board the mask is applied over the bare copper circuit connection pattern but is left off any area that is to be soldered during subsequent fabrication steps. However, because unprotected copper oxidizes and thus loses some of its solderability during the time between the fabrication of the board itself and the assembly and soldering of components thereto, it was necessary to provide a protective coating to the unmasked areas of exposed copper. The preferred material for such protective coating is generally a tin/lead material to tin the contacts for later soldering. To provide good oxidation resistance to the copper beneath the tin/lead coating, and therefore good solderability, it is preferred to have the coating of tin/lead of a thickness between about 0.0003 inches and 0.0005 inches. In the prior art method of fabricating mash-over-copper boards, this protection is applied after the solder mask is applied to the board by dipping the board into molten solder and then removing any excess solder by one of several methods. One of the most common methods is the spraying of hot oil under pressure at a temperature of about 500.degree. F over the board, thus blowing most of the solder off the copper and out of holes through the board. This technique, by providing a mask-over-copper board having a tin/lead coating over exposed copper, has proved adequate for certain applications but still suffers from numerous disadvantages. Among these disadvantages is the time-consuming nature of the solder dip process, thus slowing production of the boards. Additionally, the sudden application of 500.degree. F temperature oil to the board causes an extreme thermal shock that may result in delamination of the insulating base, delamination of the copper foil from the base laminate and other faults that tend to result in a high rejection rate. There also are dangers to the workers fabricating the boards in that the molten solder from the solder dip process and hot oil at 500.degree. F from the cleaning process may be splashed out toward the operator, thus possibly causing injury. Both the solder dip tank and the hot oil spray machine also produce large amounts of smoke, requiring both large ventilation systems and elaborate fume scrubbing systems. Finally, one of the most significant disadvantages results from the very nature of the process of removing the excess solder, some of which plugs up the holes through the boards. In spraying the solder-coated board with the hot oil long enough to unplug the holes, the desired tin/lead coating over the circuit connection points is also reduced below desirable limits, generally down to a thickness of only from 0.000020 to 0.000080, far thinner than the desired (and Mil-spec required) minimum thickness of 0.00030. With such an undesirably thin coating of the tin/lead material, the copper below frequently was not adequately protected from oxidation, thus permitting oxidation to occur, providing poor shelf life for the board and resulting in poor solderability.
The various problems encountered in fabricating printed circuit boards have thus spawned numerous experiments in developing other processes, as described in patents to Olen et al U.S. Pat. No. 3,143,484, Johnson U.S. Pat. No. 3,457,638, Degnan et al. U.S. Pat. No. 3,649,475, and Keen U.S. Pat. No. 3,554,876. However, none of these patents addressed the problem of providing a mask-over-copper printed circuit board having a desirable thick coating of protective metallic material over the copper areas not covered by the solder mask.